1. Field of the Invention
The present invention relates to a valve timing control device and, in particular, to the valve timing control device for controlling an angular phase difference between a crank shaft of a combustion engine and a cam shaft of the combustion engine.
2. Description of the Prior Art
In general, a valve timing of an internal combustion engine is determined by valve mechanisms driven by cam shafts according to either a characteristic or a specification of the internal combustion engine. Since a condition of the combustion is changed in response to the rotational speed of the combustion engine, however, it is difficult to obtain an optimum valve timing through the whole rotational range. Therefore, a valve timing control device which is able to change a valve timing in response to the condition of the internal combustion engine as an auxiliary mechanism of the valve mechanism has been proposed in recent years.
A conventional device of this kind is disclosed, for example, in Unexamined Japanese Patent Publication (Kokai) No. Hei 1-92504. This device includes:
a rotor which is fixed on a cam shaft of an engine,
a rotational transmitting member which is mounted around the peripheral surface of the rotor so as to rotate relative thereto within a predetermined range for transmitting a rotational power from a crank shaft,
chambers which are defined between the rotor and the rotational transmitting member, wherein each chamber has an advancing side, circumferentially opposed wall and a delaying side, circumferentially opposed wall,
vanes which are provided on the rotor and extended outwardly therefrom in the radial direction into the chamber so as to divide the chamber into an advancing chamber and a delaying chamber, wherein the vanes are able to move between the advancing side and delaying side walls,
a locking means for locking between the rotor and the rotational transmitting member at a predetermined relative phase, when the vane is in contact with the delaying side walls,
first fluid passages for feeding and discharging a fluid to and from the advancing chambers, and
second fluid passages for feeding and discharging the fluid to and from the delaying chambers.
In the above prior art device, when the fluid is fed to the advancing chambers via the first fluid passages and is discharged from the delaying chambers via the second fluid passages, the vanes on the rotor rotate in the advancing direction up to contact with the advancing side walls, relative to the rotational transmitting member, such that the valve opening and closing timing is advanced with respect to the crank angle. On the other hand, when the fluid is fed to the delaying chambers and is discharged from the advancing chambers, the vanes on the rotor rotate in the delaying direction up to contact with the delaying side walls, relative to the rotational transmitting member, such that the valve opening and closing timing is delayed with respect to the crank angle.
Further, when the engine is stopped, the fluid source, for example an oil pump, stops delivering the fluid. The fluid in the advancing and delaying chambers is decreased with the lapse of time. Thereafter, when the engine is restarted, there is not enough of the fluid in the chambers to hold the relative phase between the rotor and the rotational transmitting member. Therefore, each of the vanes rotates to the delaying direction and crashes against the delaying side walls of each of the chambers. The crashing sound can be bothersome to a driver and passengers. To avoid the crashing sound, the locking means locks between the rotor and the rotational transmitting member when the vane is in contact with the delaying side walls.
It is known that delaying exhaust valve closing timing makes the engine torque increase when the number of rotations of the engine is large, because fuel and air (the "charge") wants to be sucked into a cylinder of the engine by the inertia of the flow of the charge after a piston starts to move upward.
However, when the above prior art device is used to regulate a cam shaft which opens and closes intake valves, the device sets up the valve closing timing at the most delaying position which is able to suck the charge into the cylinder of the engine when the engine restarts. Therefore, the engine is not able to increase the engine torque when the engine is driven at high speed. On the other hand, if the device sets up the valve closing timing at the most delaying position which is able to increase engine torque when the engine is driven at high speed, restarting the engine is difficult because the inertia of the flow of the charge is small. As a result, although the piston starts to move upward, the intake valves open such that the charge flows in reverse from the cylinder.
In addition, if the above prior art device is used to regulate a cam shaft which opens and closes exhaust valves and the device sets up the valve closing timing at the most delaying timing, the valve overlap period is too long. This increases the amount of exhaust gas recirculation (EGR), thereby degrading the starting performance of the engine.